Its all down to preference really. 4K monitor if you can afford it. 22-27 Inch monitors are what everyone seems to buy. Choose your monitor by your budget. You can get some monitors with higher refresh rates. I believe it is called msi or something.
What do you think for a 21,5'' inch 1920x1080 Monitor ?
Im guessing it should be the same as if you were gaming on a laptop. Shouldn't be a problem, i would look at some 21.5 inch monitors on youtube to get an idea what they would look like. Should be fine.